1. Field of the Invention
The invention concerns a method for the analysis of solid samples by time-of-flight discrimination of particles liberated from this sample. It also concerns an analyzer to implement this method Time-of-flight analysis is applicable notably to mass spectrometry but can be also applied to energy analysis for particles having one and the same mass.
A method for time-of-flight analysis of sputtered particles consists in:
scanning the surface of a solid sample by a beam of particles called primary particles in order to liberate particles, called secondary particles, from the sample; PA0 ionizing the secondary particles, when they are being liberated or after they have been liberated, some of them may be already ionized during the sputtering process; PA0 accelerating them by means of an electrical field; PA0 forming a beam of secondary particles and making it travel through a path which is long enough for secondary particles of different speeds to have substantially different times of flight; PA0 subsequently discriminating among the secondary particles on the basis of their time-of-flight differences. PA0 periodically scanning the surface of a solid sample to be analyzed with a beam of particles called primary particles, to thereby liberate so-called secondary particles from the sample; PA0 ionizing the secondary particles, some of which are already charged; PA0 accelerating the secondary particles by an electrical field; PA0 forming a beam of secondary particles, and making them travel through a path which is long enough for secondary particles with different energy levels or different masses to have substantially different times of flight; PA0 then, discriminating among the second particles on the basis of their time-of-flight differences, by bringing the secondary particles with a given time of flight to a pre-determined direction, irrespectively of the place, on the sample, from which they have been liberated, in deflecting the beam of secondary particles along an angle which may vary according to the point of emission of the secondary particles, and at periodic instants having a constant phase shift with respect to the deflection previously applied to the primary particles which liberated them from the sample.
2. Description of the Prior Art
Since a sample is analyzed successively at different points, the fastest secondary particles liberated from a given point tend to catch up with the slowest secondary particles liberated earlier at another point of the sample. A known method used to prevent overlapping, in time, of particles having different starting points and different times of flight, consists in making a temporal selection by cutting up, by pulses, the primary particle beam or secondary particle beam. The drawback of this prior art method is that it lengthens the time of analysis and necessitates a complicated device to cut up either of these beams. In another known method, a continuous beam of primary particles is employed, and the electrical field used to extract and accelerate the secondary particles is cut up in pulses. The drawback of all known methods in which the primary beam is continuous but where the measurement of the flow of secondary particles is not continuous is loss of information because the beam of primary particles erodes the sample continuously while the measurements are made only at discrete instants These methods therefore have lower sensitivity of analysis than continuous measurement methods.
There are known methods of mass or energy analysis in which continuous measurements are made. These methods achieve high resolution by using geometrical selection instead of temporal selection to separate secondary particles emitted by distinct points of the sample. These methods consist, for example, in dispersing the secondary particles having different masses by means of a magnetic field. However, they generally require complicated and costly devices, which are not entirely warranted except for obtaining very high resolution. There are also known methods of analysis which achieve continuous measurement and geometrical separation by using far simpler devices such as a quadripole to perform mass spectrometry, for example. These devices are less costly and are used when low resolution suffices as their drawback is low sensitivity, these devices can work only on secondary low-energy particles, and consequently reduce the electrical field which gathers and accelerates the secondary particles. The result of this is low gathering efficency and, hence low sensitivity.
Besides, modern instruments of analysis should be capable of giving an image of the sample during the analysis, for example, to observe grains in mineralogy or in metallurgy, or else to observe implantations in a microelectronic device.